"IntarsiaCo" wrote in message
...
It is *not* hardy. Your -8 deg
C is a load of codswallop. Minimum temperature +5 deg C (41 deg F),
according to the RHS Encyclopedia

S. purpurea ssp. purpurea (the northern form of S. purpurea) is hardy to
well
below -15F/-26C; the RHS Encyclopedia is in error.

The RHS Encyclopedia does not mention S. purpurea ssp purpurea.
Neither did the OP and neither did I.
I have never known the RHS Encyclopedia to be as far wrong as this!

Franz